The power-conditioning system is an integral part of a fuel cell system. It converts the dc electric power generated by the fuel cell into regulated dc or ac for consumer use. The electrical characteristics of a fuel cell are very far from that of an ideal electric power source. The dc output voltage of a fuel cell stack varies considerably with the load current (see Fig. 9.15), and it has very little overload capacity. It needs considerable auxiliary power for pumps, blowers, and so forth, and requires consid­erable start-up time due to heating requirements. It is slow to respond to load changes, and its performance degrades considerably with the age of the fuel cell. The various blocks of a fuel cell power-conditioning system are shown in Fig. 9.16.

The dc voltage generated by a fuel cell stack is usually low in magni­tude (<50 V for a 5- to 10-kW system, <350 V for a 300-kW system) and varies widely with the load. A dc—dc converter stage is required to reg­ulate and step up the dc voltage to 400-600 V (typical for 120/240-V ac output). Since the dc—dc converter draws power directly from the fuel cell, it should not introduce any negative current into the fuel cell and must be designed to match the fuel cell ripple current specifications. A dc—ac conversion (inverter) stage is needed for converting the dc to ac power at 50 or 60 Hz (see Fig. 9.17). Switching frequency harmonics are filtered out using a filter connected to the output of the inverter to generate a high-quality sinusoidal ac waveform suitable for the load.